1. Field of the Invention
Aspects of the present invention relate to a mask for laser induced thermal imaging (LITI) and a LITI method using the same, and more particularly, to a mask for LITI and a LITI method using the same, in which an output of an optical unit is uniform.
2. Description of the Related Art
Among flat panel displays, an organic light emitting display device has a relatively fast response time of 1 ms or less, and consumes relatively low power. Further, the organic light emitting display device emits light by itself, so that it can have a wide viewing angle regardless of its size. Thus, the organic light emitting display device has advantages as a display medium of moving images, such as videos. Further, the organic light emitting display device can be manufactured at low temperatures and by a simple process based on existing semiconductor process technology. Accordingly, the organic light emitting display device has attracted much attention as a next-generation flat panel display device.
An organic light emitting device used in the organic light emitting display device may be classified into a polymer device using a wet process and a small molecular device using a deposition process depending on utilized materials and processes. One method for patterning a polymer or small molecular emission layer is an inkjet-printing method that has limitations in the type of usable materials to form organic layers other than the emission layer and requires a complicated structure on a substrate for the ink. Another method for patterning is a deposition method where the emission layer is patterned by the deposition process, which requires use of a metal mask, which makes it difficult to manufacture a large-sized device.
Accordingly, laser induced thermal imaging (LITI) has recently been developed as an alternative to the above-described patterning methods.
The LITI transforms a beam of light output from a light source, such as a laser, into heat energy, and transfers a pattern-forming material to a target substrate using the heat energy to form a pattern. The LITI requires a donor substrate having a transfer layer, a light source, and a target substrate. In the LITI, the donor substrate and the target substrate are fixed on a stage while the donor substrate entirely covers the target substrate (i.e., a receptor substrate). Then, the LITI is applied to the donor substrate, thereby completing the patterning.
When the patterning is performed by a projection method using a mask, the mask has a predetermined pattern such as a rectangular pattern.
FIG. 1A is a graph showing pattern characteristics according to positions of a substrate with respect to a mask, FIG. 1B is a graph showing a power density of a laser beam relative to positions on a substrate, and FIG. 2 is a plan view of a patterned substrate of FIGS. 1A and 1B.
Referring to FIG. 1A, a position (x, y) of an image patterned on the substrate should be in a predetermined proportion to a position (v, y) of a mask (the dotted-line). That is, the patterns of the substrate should correspond to the pattern position (v, y) of the mask in a 1:1 ratio. Accordingly, the patterns on the substrate should be formed at regular intervals and have uniform shapes. However, the actual patterned image (the solid-line) may be distorted by a projection lens.
Referring to FIG. 2, comparing a center axis 5 of the mask patterns (not shown) with a pattern 7 on a substrate 1, the image of the pattern 7 on the substrate 1 becomes increasingly distorted in the pattern 7 located towards the edges of the mask 3. That is, the pattern 7 on the substrate 1 located near the edges has a center axis that is more distant from the center axis 5 of the mask patterns, and this effect becomes more pronounced as one moves from the center towards the edges of the mask 3. Thus, an emission region 9 near the edge of the substrate 1 may not be completely patterned even when one mask is utilized.
Referring to FIG. 1B, the power density of the laser beam should be constant the dotted-line) with regard to the position (x, y) of the pattern. However, the power density of the laser beam may not be uniform (the solid-line) because of the laser beam's uneven distribution and non-uniformity due to a deformation of the laser's projection lens. That is, patterns near the edge of the mask are scanned with less laser energy than patterns near the center. Therefore, the area of the substrate 1 corresponding to the edge patterns of the mask 3 may not be completely transferred, and those patterns 7 are likely to be broken and unclear, thereby distorting an image.
As described above, the distortion of the image causes a problem in forming a uniform pattern, thereby deteriorating the quality of products, decreasing yield and increasing production cost.